1. Technical Field
The present invention relates generally to an ignition apparatus for developing a spark firing voltage that is applied to one or more spark plugs of an internal combustion engine, and more particularly, to a system configured for ion current measurement within a combustion chamber of the engine.
2. Discussion of the Background Art
So-called ion sense systems are known for detecting a combustion condition (e.g., misfire, knock). The combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. Applying a relatively high voltage across, for example, the electrodes of a spark plug just after ignition is known to produce a current across the electrodes. Such current is known as an ion current. The ion current that flows is proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above, and consequently corresponds in some measure to the ionization throughout the entire cylinder as combustion occurs. The DC level or amount of ion current is indicative of a quantity of combustion, or whether in fact combustion has occurred at all (e.g., a misfire condition). An AC level of the ion current may be used to determine whether knock exists. The ion sense approach is effective for any number of cylinders, and various engine speed and load combinations.
Known ion current sensing systems generally include a capacitor or the like configured to store a voltage. The stored voltage is thereafter used as a xe2x80x9cbiasxe2x80x9d voltage, which is applied to the spark plug to generate the ion current. One approach taken in the art involves using the voltage from a leakage inductance spike from the primary side of the ignition coil to charge a capacitor for biasing the spark plug, as seen by reference to U.S. Pat. No. 6,186,129 entitled xe2x80x9cION SENSE BIASING CIRCUIT,xe2x80x9d issued to Butler. Because of relatively good flux coupling between primary and secondary windings in xe2x80x9cpencilxe2x80x9d coils (i.e., a relatively slender ignition coil configuration that is adapted for mounting directly above the spark plug), bias voltages of approximately 100 volts are about the maximum that can be achieved (i.e., the leakage inductance spike is limited by the relatively high coupling). While biasing at about 100 volts is adequate for most combustion conditions, it is nonetheless desirable to bias at higher voltage levels under certain other conditions, for example, in highly dilute or lean conditions.
U.S. Pat. No. 6,114,935 entitled xe2x80x9cIGNITION COIL HAVING COIL CASE,xe2x80x9d issued to Oosuka et al. disclose an ignition coil extending along an axis, where the longitudinal extent of a secondary coil is about the same as the longitudinal extent of the primary coil, which is generally conventional construction for coupling primary flux to the secondary coil.
There is therefore a need to provide an ignition apparatus and an ignition system that improves upon one or more of the configurations set forth above.
An object of the present invention is to provide a solution to one or more of the problems set forth above. An increased leakage inductance spike would be required to charge the ion sense system for biasing at the increased voltage levels. One advantage of the present invention is that it provides such a configuration that increases a leakage inductance spike, which may be used by an ion sense system in providing an increased bias voltage level. This has the advantage of more effectively operating in highly dilute or lean conditions. Another advantage is that it provides an ignition apparatus having an increased, effective turns ratio (NS:NP), thereby allowing a reduction in the amount of secondary wire used, which is typically the number one raw material cost in an ignition coil. This feature reduces cost. Still yet another advantage of the present invention is that as bias voltages increase, the invention decreases waste of potential spark energy.
In accordance with the present invention, an ignition apparatus is provided that includes a central core and primary and secondary windings. The central core extends along a main axis, and the primary winding is disposed about the central core. The secondary winding is also disposed about the central core. The primary winding is extended relative to the secondary winding. That is, the primary winding has a first axial length, and the secondary winding has a second axial length that is less than the first axial length. The primary winding extension decreases flux coupling, thereby increasing a leakage inductance spike.
In a preferred embodiment, the ignition apparatus is arranged so that first and second layers thereof extend approximately the same axial length as the secondary winding, with one or more additional layers being wound to extend beyond the secondary winding at the low voltage end of the secondary winding.
In another aspect of the present invention, the above-described ignition apparatus is coupled to an ion sense biasing circuit that is coupled to the primary winding for charging thereof and is further configured to bias a spark plug coupled to a high voltage end of the secondary winding to produce an ion current indicative of a combustion condition.